In the US, breast cancer accounts for roughly 30% of all cancers in females and is the second leading cause of cancer deaths in women. For many years, the estrogen receptor has been recognized as a critical determinant of breast epithelial cell physiology, as an important clinical prognostic indicator and as an attractive target for chemotherapy in breast cancers. Humans have two classes of estrogen receptor; both exert their biological effects by directly activating transcription of specific target genes. Like all transcriptional regulators, estrogen receptor action leads to primary effects by stimulating the expression of direct target genes and also to secondary effects mediated by the biological action of direct transcriptional targets of the receptor. The downstream regulatory networks elicited by the transcriptional targets of ER constitute an important component of estrogen action. I propose to investigate a novel gene regulatory pathway constituting a secondary effect of estrogen receptor activation. We will investigate the novel human protein MTA3, a member of the MTA (metastasis associated protein) gene family. Our working hypothesis proposes that MTA3, like its well-characterized relatives MTA1 and MTA2, constitutes an integral subunit of the histone deacetylase containing transcriptional corepressor complex known as Mi2/NuRD. Preliminary characterization of MTA family members in breast cancer cell lines has determined that MTA3 expression is completely dependent on estrogen receptor function. In addition, our preliminary data indicate that MTA3 functions in a novel gene regulatory pathway that regulates invasive growth properties of estrogen-responsive cancer cell lines. I now propose to study MTA3 and its action through three specific aims. Specific Aim 1 will characterize the MTA3 gene and its transcriptional regulation by estrogens. Specific Aim 2 will provide a detailed biochemical characterization of the Mi2/NuRD complex in estrogen responsive cells with particular emphasis on changes in properties and/or subunit composition resulting from estrogen receptor action. Specific Aim 3 will identify and characterize the cohort of genes directly regulated by MTA3 using both systematic and candidate approaches. The ultimate goals of this project are (1) to provide a more complete description of the genetic program downstream of estrogen receptor and (2) to understand the contribution(s) of MTA3 to prevention of invasive and metastatic cell growth. [unreadable] [unreadable]